For wind tunnel testing of the various aerodynamic coefficients of spinning projectiles, it is necessary to use instrumented motorized spinning models of the projectiles. Present practice is to use an air turbine for spinning the models. There are however certain difficulties with this arrangement.
Because of the intrinsic physics of air turbines, the model must be fairly large in size, e.g. greater than 3.5 inches (about 9 cm.) in diameter. Because the model cross sectional area has to be very small with respect to the test section area in order to avoid blockage problems, small wind tunnels cannot be used for spinning projectile tests.
With an air turbine spinning motor, the pressure of the air line that feeds the turbine acts as an external force on the model system. This will generate error in the balance measurements. To avoid this, the air turbine is used to accelerate the model to the desired spin rate and then shut down before measurements are made. Consequently, a constant spin rate cannot be maintained. Rolling moment readings cannot be made simultaneously with sting support sweep at various angles of attack. In addition, the turbine air exhaust modifies the flow around the base of the model producing further errors in the measured data.